1. Field of the Invention
The present invention is related to a nitride semiconductor laser device having an emission wavelength in a range of 430 nm to 540 nm and particularly to a nitride semiconductor laser device improved in laser characteristics and a method for forming the same.
2. Description of the Background Art
Japanese Patent Laying-Open No. 05-243669 teaches to shift the center of a waveguide mode toward an n-type clad layer side in a laser device including an AlGaInP active layer (to reduce the light-confinement effect on the, side where the active layer is in contact with an n-type layer) by introducing an asymmetrical structure in which an optical guide layer on the n-type clad layer side is made thicker than the other optical guide layer on a p-type clad layer side. With the asymmetrical structure, it is possible to reduce light absorption in the vicinity of each end face of the laser device and increase the output level at which catastrophic optical damage occurs, thereby to increase the possible output level of the laser device.
In a nitride semiconductor laser device using InGaN material, on the other hand, there is a problem that the light confinement effect in the light-emitting layer (active layer) is inherently low in a lasing wavelength range of not shorter than 430 nm. This low light confinement effect causes decrease of the internal (and external) quantum efficiency and increase of the threshold lasing current density in the laser device and particularly brings about disadvantage for the high output operation of the device. The reason for this low light confinement effect is that the refractive index difference between AlGaN material for the clad layer and GaN material for the light guide layer generally used in the nitride-based semiconductor laser device becomes smaller as the wavelength becomes longer. In general, in order to make the refractive index difference sufficient, the Al composition ratio (atomic ratio in the III group elements) is increased in AlGaN material for the clad layer and In is added to GaN material for the light guide layer and then the In composition ratio is increased as desired. The reason for this is that the Al acts to decrease the refractive index of GaN and the In acts to increase the refractive index of GaN.
In the case of obtaining sufficient refractive index difference between the clad layer and the light guide layer by adjusting the Al composition ratio and the In composition ratio, however, the crystal lattice mismatch is increased between those layers and then there is caused generation of cracks and increase of the operation voltage in the laser device. The reason of this is that the Al acts to decrease the lattice constant of GaN and the In acts to increase the lattice constant of GaN.
As a result of the present inventors' study, on the other hand, it was found that light leakage toward the p-type layer side due to the weak light confinement effect in the light-emitting layer causes increase of light absorption due to Mg of the p-type impurity. This disadvantageously results in decrease of the external quantum efficiency and increase of the threshold lasing current density.